1. Field of the Invention
The present invention relates to a crystal substrate mounting platform for mounting a crystal substrate, such as a crystal blank, and a scratch inspection device for crystal substrates for detecting scratches optically by mounting a crystal substrate on a the mounting platform, and more particularly, it relates to a crystal substrate mounting platform and crystal substrate scratch inspecting device whereby the handling of crystal substrates can be simplified. Furthermore, the present invention relates to a bevelling inspection method and device for crystal substrates, and more particularly, to a bevelling inspection method and device for crystal substrates whereby real-time inspection is possible by means of image processing. Moreover, the present invention relates to an inspection method for crystal substrates, and more particularly, to a method whereby scratches and the bevelling state of crystal substrates can be inspected region by region.
2. Description of the Related Art
Crystal oscillators are fabricated from crystal blanks, but if there is even the slightest scratch in the crystal blank, then the crystal oscillator will be defective, and therefore scratch detection in crystal blanks is extremely important.
Until now, defects in crystal blanks have been inspected visually by people, by since this operation depends on human sight, it is extremely difficult to detect scratches of the order of several ten .mu.m or less. Moreover, since this operation continues for a long period of time, the operator becomes extremely tired and consequently stable inspection results cannot be obtained, so inspection is duplicated by two or three people. Therefore, a scratch inspecting device based on image processing has been sought.
A conventional scratch inspection device based on image processing is disclosed in Japanese Unexamined Patent 7-103905, for example.
As shown in FIG. 22, in this device, light is shined onto an object under inspection 60, such as a crystal blank, from three directions by providing illumination sources 61.about.63 in positions mutually separated by 90.degree., and CCD cameras 64, 65 are placed between the illumination sources. These CCD cameras 64, 65 and the illumination sources 61.about.63 are placed in positions having an angle of 45.degree. with respect to the horizontal face of the object under inspection. To inspect scratches, one of the cameras and one of the two illumination sources on either side of this camera are turned on at the same time, and the other illumination sources and cameras are turned off. Four images are taken by the cameras 64, 65 by implementing the four on and off combinations, thereby covering a 360.degree. detection range. Each image is recorded in image input means 66, and judgement processing is implemented for each of these image signals, respectively, by feature extracting means 67 and quality judging means 68, to detect flaws, such as scratches which are independent of the direction of fracture, and the like.
However, the following problems are associated with the conventional technology described above.
1. Since the illumination and image-capturing operations are implemented obliquely using a plurality of illumination sources and cameras, equal quality in terms of focussing and light conditions setting cannot be ensured for all images, so accurate detection can be made. PA1 2. Since the cameras are set in an oblique direction, the image becomes elliptical, so accurate dimensions and measurements cannot be obtained. In order to determine accurate dimensions and measurements, complex correctional processing is necessary. PA1 3. Complex control is required for switching the cameras and illumination sources. Furthermore, in order to inspect one object, it is necessary to take a plurality of images, so the detection algorithm is complex and the image processing speed cannot be raised. PA1 4. Since the cameras and illumination sources are both positioned at an angle of 45.degree. with respect to the horizontal face, there is the risk that the background pattern of the mounting platform on which the object under inspection is mounted will be input. If the background pattern of the mounting platform is input, the SN ratio with respect to images of scratches or defects will deteriorate, thus making detection more difficult.
Therefore, in order to resolve the aforementioned problems, a scratch inspection device, whereby scattered light is shined onto the face of the object under inspection, such as a crystal blank, within an illumination angle of .+-.30.degree., from all sides of the perimeter of the object, has been investigated (Japanese Unexamined Patent 9-288063). By this means, since a dark field of view is illuminated from all sides of the perimeter of the object under inspection, and furthermore, since the illumination light is scattered light, reflected light reflected by scratches or edges is emphasized, and scratches or edges only stand out clearly in the image. Since there is no illumination at an angle greater than 30.degree. with respect to the front or rear surface of the object under inspection, light which simply passes through the object under inspection and is reflected at the surface thereof does not form an image and therefore the image of the object under inspection as a whole forms a shadow and is not captured. The light forming an image is only the reflected light which is reflected either by scratches present in the object under inspection or the sides (edges) of the object under inspection. By image processing of this reflected light, scratches can be identified readily.
On the other hand, bevelling (chamfering) the outline (perimeter) of the crystal blank is used as a method for improving the characteristics of a crystal oscillator. In general, a cylindrical barrel system capable of high-capacity processing is used for bevelling. As illustrated in FIG. 23, this involves introducing a grinding material 42 into a cylindrical barrel 41 along with a plurality of crystal blanks 1 and grinding by causing the cylindrical barrel 41 to rotate. In order to improve crystal oscillator characteristics with good reproducibility, it is necessary to increase the accuracy of bevelling dimensions by means of the aforementioned method to achieve lateral and vertical symmetry.
In the aforementioned cylindrical barrel system, the bevelling state varies depending on the number of crystal substrates introduced into the barrel, the type of grinding material, the speed of revolution, and the like, and even if these conditions are kept uniform, this does not necessarily mean that the same bevelling state will always be achieved. Crystal substrate manufacturers rely on their own know-how to a large degree, but since this know-how is a variable factor, it cannot be regarded as an ideal approach, and currently uniform accuracy in bevelling dimensions cannot be maintained at all times and unexpected factors also arise. In order to improve accuracy in bevelling dimensions, a bevelling inspection method is required, which is capable of evaluating the state of bevelling of the crystal substrate surfaces, and feeding these results back to the bevelling process.
Since the bevelling process is achieved by applying extremely small scratches, a standard light field illumination method will not produce any significant difference between bevelled and unbevelled sections, so the bevelling state of the crystal substrate surfaces cannot be inspected by means of visual inspection by an operator, or by an image processing technique. Therefore, conventional inspection of bevelling has relied unavoidably on physical methods, such as (1) bevel measurement or (2) projection.
(1) Bevel measurement method
This method uses a laser height measuring device, and involves vapour deposition of reflective silver film onto the rear face of the crystal blank, whereupon a laser light source is shined onto the surface of the crystal blank and swept (scanned) in a diametrical direction (linear direction), and the level on straight lines is measured from the reflected light. By this means, the height of the surface in a linear direction of the crystal blank can be measured in a continuous fashion, and the state of bevelling can be determined from this surface height.
(2) Projection method
In this method, carbon powder (black) is coated onto the surface of a crystal blank, semi-transparent film or thin paper is pressed thereonto, and the carbon powder is transferred to the film or thin paper. The film bearing the transferred carbon is then projected in an enlarged state onto a screen. Since no carbon adheres to the unbevelled regions, the sate of bevelling can be observed visually from the state of transfer of the carbon powder.
On the other hand, if scratches in crystal blanks are inspected by image processing, the image signals read in are digitized in order to emphasize scratches, but it is necessary to set a threshold value for this digitization operation. The threshold value is an important parameter in determining the quality criteria applied to the inspection results. Conventionally, this threshold value is set as a uniform value for a crystal blank, regardless of the position on the blank.
Since a crystal blank cannot be used if it is scratched, a total product inspection is required rather than a sampling inspection. In view of the dramatic increase in demand for crystal blanks in recent years, it has become essential to achieve automation and increased speed in the aforementioned scratch inspection devices currently under investigation. In this case, a bottle-neck is caused by the mounting and dismounting of crystal blanks on the substrate mounting platform. Specifically, since the crystal blanks and substrate mounting platform both have mirror finishes, during mounting an air layer of almost uniform thickness is formed between the crystal blank and the substrate mounting platform, making the crystal blank liable to slip over the substrate mounting surface or causing the crystal blank to adhere to the substrate mounting surface once the air layer has been expelled after mounting. This phenomenon is particularly marked in cases where the substrate under measurement is 30 .mu.m.about.500 .mu.m thick, or in crystal blanks with edges or diameter between 3 mm.about.50 mm long.
Therefore, when a crystal blank conveyed by a conveyor robot arm of an automated device is mounted in the substrate mounting surface, the crystal blank may slide over the mounting platform, and the mounting position will be unstable, thus impeding accurate measurement. Moreover, after mounting, the crystal blank may be bonded tightly to the substrate mounting surface, preventing smooth a pick-up operation by the conveyor robot arm after inspection, and thereby obstructing the achievement of increased speed and automation in the inspection process.
Therefore, an operation whereby scratching is applied to the polished glass form of the mounting surface of the substrate mounting surface by applying sand thereto, such that the crystal blank does not slide or adhere tightly, has been investigated. However, if scratches are simply applied to the mounting surface to form small indentations, then slightly sharpened projecting sections will form at random on the mounting surface, which may break readily upon contact with a crystal blank, and even if quartz glass of similar hardness to the crystal is used for the mounting platform, then within several hours the tips of these projecting sections will wear away and will become difficult to distinguish from scratches on the crystal blanks, thereby impeding detection of scratches. Moreover, dirt may enter into the small indentations and this dirt is not readily removed, even by wiping, and if it adheres to the crystal blank, then it may be difficult to distinguish from scratches.
On the other hand, there have been the following problems with the conventional bevelling inspection methods described above.
(1) Level measurement method
Although the height of the polished surface can be measured accurately, since data is only obtained for the straight lines in which the crystal blank is scanned, it is not possible to inspect bevelling with respect to the whole surface of the crystal blank. If the scan in the horizontal direction is shifted in the vertical direction, surface data will still be obtained, but data omissions corresponding to the pitch width in the vertical direction will inevitably occur. Therefore, it is difficult to inspect the whole surface of a crystal blank. Moreover, since a process for vapour deposition of silver onto the rear surface is required, the inspection becomes a sampling process, which takes up time, and furthermore, the inspected samples cannot be used.
(2) Projection method
Although the whole surface of the crystal blank can be inspected, since it is necessary to take a sample crystal blank, and perform the transfer and projection operations, the inspection becomes a sampling process, which takes a very large amount of time. Since the method is based on visual observation, inspection data is not obtained.
In this way, neither of the methods (1) or (2) above allow inspection in real time, and both take up a large amount of time. Furthermore, since an integrated inspection method cannot be established, standardization with bevelling inspection cannot be achieved either. Moreover, since suitable data cannot be obtained, a feedback operation, which is effective in bevelling process technology, cannot be implemented.
Furthermore, since a batch method handling large quantities is used for the bevelling process, there are differences in the bevelling state between crystal blanks, and variations are also produced in subsequent etching processes using acid treatment. Regardless of this, as no inspection standards are set, it is difficult to determine tolerances and satisfactory items may be treated as defective, whilst defective items may be treated as satisfactory.
On the other hand, in conventional scratch inspecting methods, scratches are inspected by applying uniform inspection standards across a whole crystal blank, irrespective of the point of scratch inspection. However, crystal blanks forming the object under inspection usually incorporate some variation acquired during the manufacturing stages and they cannot be produced in a completely uniform manner, in addition to which, in some cases, more lenient standards can be applied depending on the region within the crystal blank. Therefore, if scratches are inspected according to a single inspection standard, instances where a satisfactory item is judged to be defective or where a defective item is judged to be satisfactory may occur. Furthermore, even if scratch inspection data based on image processing is obtained, since information relating to the inspection point is not entered, it is effectively impossible to acquire data for standardization of inspection or analysis control. This problem is not limited to scratches, but also arises in relation to inspection of the state of bevelling (chamfering).